Apple Watch shortened activity recordings

There’s a known Apple bug where if you start a recording while the Watch is still connected with your phone, the route on the map appears to start later and the total distance is shorter compared to the Watch's unsaved activity summary. What happens is that a few points get recorded into the file that are pulled from the Phone’s GPS instead of the Watch. When the Watch goes outside and the phone stays at home, it takes the Watch a long time to realize that it is disconnected and that it needs to start getting its own location.

To avoid this in the future, start your recording away from your phone if you’re going to leave it behind.